The molecular structures of metallic and ceramic materials differ substantially from each other. In the metallic bond, the electrons orbit the atomic nuclei disorderly and with comparatively to bonding force. Ions, for example in the body environment, separate constantly from this “loose” structure; a variety of chemical reactions are possible.
In ceramic molecules, the electrons in the ceramic bond follow precisely predefined paths, the so-called directed electron orbitals. Their bonding force is very high; the molecules are extremely stable. Therefore, no formation of ions takes place and chemical reactions are virtually impossible.
The extremely stable ceramic bond almost excludes plastic deformation of the material. This effects, on the one hand, the desired extremely high hardness, but, on the other, it results in relatively high brittleness. However, with the correct material design, it is possible to achieve high hardness and high ductility at the same time.
Material science distinguishes between fracture strength and fracture toughness. Fracture strength is the maximum mechanical stress a material resists without breaking. Fracture toughness, or crack initiation toughness, describes the resistance of a material against the onset of crack propagation. Ceramic materials which have very high fracture strength are today already in use in medical technology. Some of these materials have in addition extremely high fracture toughness. Such materials have a much better resistance against the onset of cracks than other ceramics and can retard the growth of the crack.
This property is based on reinforcement mechanisms. The first reinforcement mechanism is owed to the embedded tetragonal zirconium oxide nanoparticles. These particles are individually distributed in the aluminum oxide matrix. They generate local pressure peaks in the region of the cracks and counteract crack propagation in this manner.
The second reinforcement mechanism is achieved through platelet-shaped crystals which likewise form sporadically in the oxide mixture. These “platelets” deflect potential cracks, disperse crack energy and thus dissipate energy. Both functions with such materials also allow constructing component geometries which were not achievable in the past with ceramics.